The first x-ray images were made by exposing photographic film to an area beam of x-ray radiation after it had passed through a patient. Photographic film is still the medium of choice for many radiographic procedures, particularly where high image resolution is required. The photographic film may be coupled with a phosphor screen, which enhances the film sensitivity to x-ray radiation by converting the x-ray radiation into visible light.
Often it will be necessary for a doctor to view an x-ray image in real-time while performing a surgical procedure such as a cardiac catheterization. In these circumstances, the x-ray film is replaced with an image intensifier and a television camera. The x-ray striking a phosphor screen at the front of the image intensifier, produces a faint light image which is intensified by the image intensifier and read by the television camera. The use of the image intensifier permits a lower dosage of x-rays commensurate with the need to continuously expose the patient with radiation while a real-time image is acquired.
In certain circumstances, it is desirable to convert an x-ray image into a digital representation for processing by a computer. The digital representation of the image (digital image) may be processed, for example, to enhance edges in the image or correct for distortion of the image intensifier. Certain techniques such as digital subtraction angiography require that two images of a patient, one taken with and one taken without a contrast agent injected into the patient, be subtracted from each other. This subtraction may be done easily with digitized images.
Digital images may be obtained by scanning conventional photographic film or by using a photostimulable phosphor plate which is exposed like film then taken to a reader to be scanned and digitized. Alternatively, the electrical signal output by the television camera on an image intensifier/television camera system, may be converted directly to a digital signal through the use of a high speed analog to digital converter.
With improvements in the fabrication techniques for constructing large area integrated circuit arrays (such as are used in LCD-type computer displays) there has been considerable interest in constructing a large area solid state x-ray detector, that provides a digital signal directly to processing equipment. One such detector design described in U.S. Pat. No. 4,996,413 issued Feb. 26, 1991 to the same assignee as that of the present invention and hereby incorporated by reference, employs an array of cells each comprised of a photodiode and thin film transistor arranged in columns and rows beneath a phosphor. An intrinsic capacitance associated with each diode is first charged and then the array is exposed to x-rays. X-rays striking the phosphor produce light photons, which then strike the photodiodes, causing charge to be lost from their intrinsic capacitances. After a period of exposure, charge is restored to the photodiodes. The amount of charge restored to each photodiode indicates the x-ray dose received by each photodiode. An electrical signal indicating the restored charge is digitized and stored as a digital image.
In order to provide suitable spatial resolution, a large number of photodiodes are employed. The wiring necessary to connect each photodiode to the necessary charging and measuring circuitry, is reduced by connecting the photodiodes to individually addressable columns and rows. Specifically, each photodiode is connected through a solid state switch to a column conductor common with all the other photodiodes in a given column. The photodiodes may therefore share wiring by being read-out one at a time through time division multiplexing. Specifically, a single column conductor provides a charging current to all photodiodes in a given column and is connected to a separate measuring circuit for that column, which can quantify the amount of charging current provided to the photodiodes of that column. Control terminals of the solid state switches which when asserted, allow current to flow to the photodiodes of one row, are connected to row conductors common for all the diodes of a given row. Thus, after exposure of the photodiodes, the photodiode array may be scanned by selectively asserting one row conductor to charge all the photodiodes in a given row. Because only one photodiode of that row is connected to each column conductor, the amount of current flowing through the column conductor when a given row conductor is asserted is related to the recharging of a single photodiode. This process is repeated with each row conductor successively asserted until each of the photodiodes is recharged and the amount of restoring charge required is measured.
Attached to each column conductor (so as to measure the charge passing into the column conductor) is an integrator, which integrates the current flowing into the column conductor over the time that each row is asserted to produce a total charge measure. At the end of integration, prior to the charge measurement, the integrator must be allowed to "settle" for a short period of time to remove the effect of noise spikes, caused by the switching of the solid state switches coupled to the column conductors by the crossing row and column conductors. After the charge measurement, the integrator must be reset prior to the next row being measured.
As a result of the non-single crystalline structure of amorphous silicon, a large density of defect states exists within the photodiode. These defect states trap electrons and holes and release them with a time constant determined mainly by the energy level of the defect state, in some cases much longer than a fluoroscopic frame time. For simplicity, we will refer only to trapped electrons throughout this document, but it should be understood that holes can be trapped in a like manner and the same mechanisms apply to holes. Therefore, whenever the electric field within the photodiode is perturbed either by electrons generated by light from an x-ray exposure or by the bias voltage being varied, trapped electrons within the photodiode are redistributed among these defect states, generating a detrapping current with a long time constant at the photodiode terminals.
Under certain circumstances, such as when a low dose fluoroscopic image sequence immediately follows a high dose radiographic image, the number of the detrapping electrons is significant compared to the fluoroscopic image signal levels. Since the release of these trapped electrons takes typically much longer than a fluoroscopic frame time, and the image of the detrapping electrons will appear in the form of the high dose radiograph image, the subsequent fluoroscopic images will appear corrupted with a slowly decaying ghost image of the previous radiograph. This phenomena is typically referred to as photodiode lag.